feat: add proxy support via child process spawning

When proxy environment variables (https_proxy, HTTPS_PROXY, http_proxy,
HTTP_PROXY) are detected and NODE_USE_ENV_PROXY is not already set to
"1", the action spawns a child process with NODE_USE_ENV_PROXY=1 to
enable Node.js native proxy support.

- Add lib/run-with-proxy.js shared utility for both main.js and post.js
- Update main.js and post.js to use runWithProxy() wrapper
- Add tests for proxy spawning, child error handling, and already-enabled path
- 100% code coverage maintained

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

lib/run-with-proxy.js

@@ -0,0 +1,44 @@
+// @ts-check
+
+import { spawn } from "node:child_process";
+
+/**
+ * Wraps a function to automatically enable Node.js proxy support when proxy
+ * environment variables are detected. If proxy env vars are set but
+ * `NODE_USE_ENV_PROXY` is not `"1"`, spawns a child process with
+ * `NODE_USE_ENV_PROXY=1` to enable native proxy support.
+ *
+ * @param {() => Promise<void>} run
+ * @returns {Promise<void>}
+ *
+ * @see https://github.com/nodejs/node/blob/4612c793cb9007a91cb3fd82afe518440473826e/lib/internal/process/pre_execution.js#L168-L187
+ */
+export async function runWithProxy(run) {
+  const httpProxyEnvVars = [
+    "https_proxy",
+    "HTTPS_PROXY",
+    "http_proxy",
+    "HTTP_PROXY",
+  ];
+  const nodeHasProxySupportEnabled = process.env.NODE_USE_ENV_PROXY === "1";
+  const shouldUseProxy = httpProxyEnvVars.some((v) => process.env[v]);
+
+  if (!nodeHasProxySupportEnabled && shouldUseProxy) {
+    return new Promise((resolve, reject) => {
+      const child = spawn(process.execPath, process.argv.slice(1), {
+        env: { ...process.env, NODE_USE_ENV_PROXY: "1" },
+        stdio: "inherit",
+      });
+      child.on("exit", (code) => {
+        process.exitCode = code;
+        if (code !== 0) {
+          reject(new Error(`Child process exited with code ${code}`));
+        } else {
+          resolve();
+        }
+      });
+    });
+  }
+
+  return run();
+}